1. Field of the Invention
The present invention relates to a process for the determination of the concentration of a substance (i.e., a solute) dissolved in a solvent by means of an osmometer, and to an apparatus for the execution of the process.
2. Description of the Art
Processes for determining the concentrations of solutes and apparatus for the implementation of the processes are described in U.S. Pat. No. 4,706,495 (which corresponds to German Laid Open Patent Application No. 35 25 668) and in German Patent Application No. P 37 06 361. In the processes described therein, an artificial osmosis cell is used as a sensor and contains a solution of a nonpermeating osmotic substance as a sensor. In the presence of a pure solvent (e.g., water), a constant hydrostatic pressure builds up in the cell, which is measured with an electronic pressure measurement sensor. In the presence of a measurement solution in a container, as a result of an osmotic volume flow out of the cell into the container, a pressure is obtained which changes with time, and which, in general, exhibits a two-phase curve, namely, one phase due to an osmotic water flow and another phase due to the flow of solutes. The concentration of dissolved substances is then determined from the curve.
This technique can be used principally for the determination of the concentrations of low-molecular substances in mixtures, whereby the selection of the membrane and coupling with chemical reactions makes possible a high degree of flexibility in its adaptation to defined measurement problems. For example, the concentration of solvents in waste water from the chemical industry can be measured using this process, as can the concentration of pollutants in solutions. Similarly, the decomposition products in bioreactors used in biotechnology processes can be directly determined. Other areas of application are the determination of alcohol concentration during alcoholic fermentation and the measurement of blood alcohol levels.
Such osmometers are also used to detect or to measure transport processes in plants and in the soil. They can also be used to simulate moisture stress situations, such as those which occur during drought or frost. The present invention is also intended for that field of application. With regard to related processes and the osmometers appropriate to them, reference is made to the above-referenced art.
For the determination of the concentration of a substance, both solutions with nonpermeating substances and those with permeating substances can be used. In the osmometer solution, a nonpermeating substance is used so that as a result of the osmotic pressure difference across the membrane in the osmosis cell, a hydrostatic working pressure builds up, which can be adjusted by using corresponding concentrations, such that, in the subsequent determination of the substance in the measurement solution, these differential pressures can be easily measured. For the adjustment, a reference solution is introduced into the measurement chamber which appropriately contains a pure solvent, e.g., water. After the determination of the working pressure P.sub.0 in the osmosis cell, the reference solution placed in communication across the membrane is replaced sufficiently rapidly by the solution containing the substances to be determined, whereby the minimal pressure P.sub.min is established. Since this minimal pressure is determined by the concentration of the permeating substance in the solution, the exchange of the two solutions must occur very rapidly in relation to the half-time for the flow of the pure solvent through the membrane, which can be on the order of a few seconds.
The above-mentioned U.S. Pat. No. 4,706,495 also discloses alternative processes. The concentrations of the permeating and nonpermeating substances in the solution can also be determined on the basis of the pressure curve.
To be able to measure the concentration of certain substances in very small quantities, German Patent Application No. P 37 06 361 discloses a differential pressure process and apparatus, according to which a reference solution is selected which contains the same substances as in the solution to be measured, and in which the difference in the concentration between the reference and the osmometer solution is not greater than 70 mOsm. The process can also be refined by coupling it with a chemical reaction in the cell, e.g. an enzyme reaction, whereby a particularly high selectivity of the measurement system is achieved.
U.S. Pat. No. 3,635,075 discloses a so-called "Hepp" osmometer, in which the deformation of a pressure transformer membrane or a meniscus occurring as the result of a water flow is measured. To compensate for interferences by hydrostatic pressure differences on both sides of the membrane, there is a bypass, so that only the osmotic pressure is measured. The measurement principle employed is based on a change of the pressure or volume of an osmosis cell, i.e., water flows and volume changes take place in the osmosis cell.
In U.S. Pat. No. 4,706,495 (which corresponds to German Laid Open Patent Application No. 35 25 668), it is suggested that, with regard to a general control of the measurement head, not only for rigidity of the osmotic cells, but also in order to allow determination of the modulus of elasticity of the osmotic cell, it may be appropriate to provide an apparatus for effecting a controlled volume change of the osmotic cell. With such an apparatus, for example, the volume of the osmotic cell may be varied, within certain limits, by inserting and/or extracting a control rod. The resulting changes can be measured via a micrometer. With a closed osmotic cell, the resulting pressure changes may be detected by means of a pressure sensor, and therefore, with a known volume, the elasticity of the osmotic cell may be determined.
It has been determined that the processes and devices described above have disadvantages in terms of the measurement time and the separation of permeating components. In particular, substances whose permeability differs only slightly from that of the solvent are difficult to measure. Moreover, these processes have the disadvantage that the measurement times for the determination of the concentration are too long (25 minutes in the best case and several hours in the worst case for certain solutions).